Water plays an important role in shaping the modern world. Cities are built on water. Humans rely on water for cooking, drinking, washing, transportation, trade, energy, irrigation (watering crops), and recreation.

The use of water in the modern world has also created problems. Population growth and advancements in technology threaten the world’s water supply. Overfishing and pollution stress many of the world’s seas, and shortages of water stress human populations in arid (extremely dry) lands.

Cities and ports

Most cities are located beside water, Coastal areas in particular boast large cities. Eight out of the top ten most populous cities in the world lie on the coast. Nearly 44% of the world’s population lives within 100 miles (161 kilometers) of a coast.

Coastal cities grow because ports are an integral part of modern life. A port is place where people and merchandise can enter or leave a country by boat. Ports are essential for trade, or the movement of materials in exchange for money.

Cities that are not located on the coast are usually located on some other body of water, such as a lake or a river. Cities need large supplies of freshwater for drinking or irrigation. Cities get most of their freshwater from the nearest river or lake.

Even most coastal cities are located where rivers flow into the ocean. These rivers provide coastal cities with a supply of freshwater. Goods delivered to a coastal city’s port may also be shipped further inland on the river.

Modern cities have factories that make goods required in today’s world. These factories often produce pollution that makes its way into the water supply, whether through inadequate storage or treatment facilities, or as a direct source of pollution through dumping industrial wastewater. Cities must also dispose of raw sewage.

In most developed countries, sewage is treated and returned to the water supply. This has minimal effect on the environment. In most developing countries though, raw sewage is pumped back into rivers, lakes, and the ocean. Both pollution and sewage can kill animals, plants, and microorganisms living in the water.

Exploration

In past centuries ocean exploration meant sailing the open seas in search of new lands. Today ocean exploration usually involves exploring below the surface of the ocean. Although oceans cover nearly two thirds of Earth’s surface, little was known about what lay below the surface until the twentieth century.

The Mid-Atlantic Ridge, a mountain range that stretches the length of the Atlantic Ocean, was not discovered until 1952. The Marianas Trench, the deepest point in the Atlantic Ocean, was not discovered until 1951.

Several technological advances made underwater exploration possible. Humans cannot dive far underwater because of a lack of air, cold temperature, and the extreme pressure underwater. Submarines called bathyscaphes are required to go the deepest parts of the oceans. Bathyscaphes protect humans and equipment from the cold temperatures and extreme pressure of the ocean depths.

The deepest point of the ocean is the Mariana Trench, at 35,802 feet (10,912 meters) below sea level, and almost 7 miles (11 kilometers) below the ocean’s surface. At this depth, the water pressure is nearly 16,000 pounds (7,257 kilometers) per square inch.

Discoveries at the bottom of the ocean surprised scientists. Scientist had long assumed that all living organisms depended on sunlight for life. Plants require sunlight for photosynthesis, the process where light, water, and carbon dioxide are converted into food. Many animals then rely on plants as basis of their food chain (the relationship between plants and animals where one species is eaten by another).

In the 1970s scientists found small communities of organisms living in complete darkness. These organisms depend on hydrothermal vents for survival. Powered by volcanic activity, hydrothermal vents are ocean-floor geysers (hot springs) that spew out a fluid rich in chemicals and minerals.

Some of the fluids from hydrothermal vents are nearly 750°F (399°C). The animals that live near these vents rely on chemosynthesis for survival. Chemosynthesis is the use of chemicals, rather than sunlight, for the production of energy.

Irrigation

More freshwater is used for irrigation than for any other purpose. Irrigation usually involves pumping or diverting water from a river or lake that may lie far away. The water is then sprayed over crops.

More than half of all freshwater usage worldwide goes toward irrigation. In the United States, 40% of freshwater usage is for irrigating over 51 million acres of cropland. Over 130 million gallons of water are used for irrigation in the United States every day, enough to fill 144 Olympic-sized swimming pools. Farms in the western United States use most of this water.

Over the last century methods of irrigation have improved. As a result humans are growing more crops than ever before. This is a useful advancement given the world’s growing population and increasing need for food.

Irrigation is a necessity but the process has some negative impacts on the environment. First, only about half of all water used for irrigation is returned to the water supply. The rest evaporates. Second, irrigation can carry pesticides into the water supply.

Pesticides are chemicals that are used to kill or repel insects, rodents, and other pests. These pesticides can build up in the water supply and harm ecosystems (communities of organisms and their environment). Third, areas that use seawater for irrigation run the risk of depositing too much salt onto the land.

Salt is usually removed from water through a process called desalinization, yet some salt may remain in the water. Soil with accumulated salt prevents crops from growing. About 10% of soil in the world’s irrigated land now contains too much salt.

Fourth, irrigation can place heavy demands on a freshwater source, such as a river, and deny the river’s resources to those downstream of the point where water is removed for irrigation. Because of irrigation and other overuse, the Colorado River in the western United States and Mexico no longer flows into the Gulf of California.

Water systems

Freshwater has two primary household uses: drinking and sanitation. Sanitation uses include showering, washing clothes, flushing the toilet, and washing dishes. These are important activities to control the spread of diseases.

Clean drinking water is also important for preventing disease. Freshwater that comes directly from a river or lake is not usually clean enough to drink or use for sanitation. Water from a river or lake can contains disease-causing microorganisms.

Water must be treated to remove these microorganisms. The treatment process occurs at a water treatment facility. The treatment process purifies water by removing microorganisms, dirt, and sediment (particles of sand, soil, and silt) from water. This process improves the purity of drinking water.

Once water has been used in the home the water then goes to a wastewater treatment facility. Wastewater treatment facilities remove most of the waste from water. Wastewater facilities usually do not purify the water well enough for it to be used as drinking water. Treated wastewater, called effluent, is often used for irrigating crops or cooling power plants. Effluent that is not used is returned to a lake or river.

Most people in the United States are accustomed to having clean water whenever it is needed. In many countries however, clean water is not available. Over 1 billion people do not have access to potable water (water safe to drink). About 2.4 billion people lack proper sanitation facilities.

This lack of drinking water and poor hygiene causes the deaths of millions of people every year from cholera, the disease responsible for more deaths than any other worldwide. This problem could become worse as the population in developing countries increases.

Trade and transportation

Ships began to replace their steam engines with diesel in the twentieth century. A diesel engine burns oil products to create the energy needed to turn a ship’s propellers. In the 1950s scientists developed nuclear powered ships and submarines that could remain at sea for months without refueling.

Due to environmental concerns and cost, nuclear power is only used on military vessels. The ability of a submarine to remain underwater for months at a time is a great military advantage.

For centuries shipping cargo over the oceans was cheaper than shipping over land. Shipping by boat has become even cheaper in the modern world. Large ships that could travel faster and carry more cargo led to an expansion of trade. As shipping costs decreased, the cost of products went down. Today, nearly 90% of the weight of all cargo is shipped by boat.

Travel

During the nineteenth century steam-powered ships made travel more popular. A steamship could cross an ocean in a matter of days compared to weeks for a sailboat. Passenger ships carried tourists and immigrants from continent to continent.

By the 1950s and 1960s air travel had replaced ships as the primary form of ocean transportation. The passenger ship industry responded by making their ships a vacation instead of simply transportation. Millions of people take cruises to exotic locales every year.

Around the world, many people visit bodies of water. The shores of rivers, lakes, and oceans are favorite places for vacations. Venice, Italy, a city built upon canals, is one of the world’s most popular tourist destinations. In Venice, the canals are the city’s roadways. Instead of cars, busses, and taxis, and ambulances, people use boats for transportation.

Sometimes water makes travel more difficult. To cross the English Channel, a narrow body of water between England and France, people used to rely on ferries. Ferries are large boats that transport people, cars, and trucks.

Frequent storms often delayed ferry travel. In 1994 a tunnel opened that runs beneath the channel seafloor. The Channel Tunnel (or Chunnel) allows trains to rapidly transport cars and people between France and England in any weather.

Hydroelectric power

Humans have learned to harness the energy of river water and use it to generate electricity, called hydroelectric power. Hydroelectric power is electricity generated by an electric power plant whose turbines (devices that converts the flow of a fluid into mechanical motion) are driven by falling water. About 10% of the electric power generated in the United States comes from hydroelectric power.

Hydroelectric power production requires the construction of a dam. A dam is a barrier that holds back the water on a river, forming a lake called a reservoir. The water from the reservoir is released through gates in the dam.

The water turns turbines as it flows rapidly through the dam. The turbine turns generators, or machines that produce electricity. The water then comes out the other side of the dam and flows downriver.

Religion and popular culture

Because water is necessary for life, many of the world’s religions use water in their rituals. Hindus believe the waters of the Ganges River in India are sacred. Christians use water for baptisms. Muslims bathe their feet before entering a mosque. In Japan Shinto shrines feature a tsukubai, a large bowl of water for followers to wash their mouths and hands before entering.

Water is also part of popular culture. The main ingredient in all popular soft drinks is water. Drinks brewed in water, such as coffee and tea, are favored around the world. The Japanese, Chinese, and several indigenous cultures developed special ceremonies for drinking tea.

In Europe and the United States coffee shops and cafes are popular gathering places. Many gathering places and public parks feature fountains as works of art and areas for reflection.

Beaches and other waterways are popular settings for educational and entertaining books, movies, and television programs. The “Jaws” series of books and movies in the 1970s and 1980s created intense public interest and misconceptions about sharks, most notably the myth that sharks seek out people to kill for food.